1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup apparatus including the same, and more particularly, to a zoom lens system suited for use in a camera for broadcast, a video camera, a digital camera, and a film camera.
2. Description of the Related Art
In recent years, a zoom lens system with a large aperture ratio, a high zoom ratio, and a high optical performance is desired for use in an image pickup apparatus, such as a television camera, a film camera, a digital camera, or a video camera. A positive lead type four-unit zoom lens system has been known as the zoom lens system with a large aperture ratio and a high zoom ratio. In the four-unit zoom lens system, four lens units are provided in total, and one of the lens units located closest to an object side has a positive refractive power. The four-unit zoom lens system includes a first lens unit for focusing, a second lens unit having a negative refractive power for variation of magnification, a third lens unit having a positive refractive power for correcting shift of an image plane due to the variation of magnification, and a fourth lens unit having a positive refractive power for imaging, which are arranged in the stated order from the object side to the image side. As an example of this type of four-unit zoom lens system, four-unit zoom lens systems having a high zoom ratio are known (see U.S. Pat. No. 6,940,656, Japanese Patent Application Laid-Open No. 2004-264459, and U.S. Pat. No. 7,505,213). In U.S. Pat. No. 6,940,656 and Japanese Patent Application Laid-Open No. 2004-264459, a zoom lens system with a high zoom ratio of about 96 is disclosed. In U.S. Pat. No. 7,505,213, a zoom lens system having a high zoom ratio of about 50 is disclosed.
The above-mentioned positive lead type four-unit zoom lens system may support a large aperture ratio and a high zoom ratio relatively easily. However, if the zoom ratio becomes as high as about 100, a large amount of variation of various aberrations may occur, leading to difficulties in achieving a high optical performance over the entire zoom range. In particular, a large amount of lateral chromatic aberration and longitudinal chromatic aberration may occur at the zoom position close to the telephoto end. Therefore, it is important to effectively correct chromatic aberrations in a secondary spectrum as well as those in the primary spectrum to form a high quality image. In the positive-lead type four-unit zoom lens system, in order to achieve a high zoom ratio, to effectively correct chromatic aberrations, and to achieve a high optical performance, it is important to appropriately set the configuration of lenses of the third lens unit for correcting a shift of an image plane due to the variation of magnification. If the lens configuration of the third lens unit is not appropriately set, a large amount of variation of aberrations, such as chromatic aberration, spherical aberration, halo, coma, and color difference of spherical aberration, may occur during zooming, which results in difficulty in achieving both a high zoom ratio and a high optical performance at the same time. In particular, in a case where the zoom ratio is increased to as high as about 100, longitudinal chromatic aberration may increase at the telephoto end, and further residual aberration in the secondary spectrum may be increased in amount, which may be difficult to be corrected appropriately.